This invention relates to the use of equalizers for receivers in the digital television broadcasting industry.
Digital technology is rapidly becoming the platform of choice for many types of communication. Sophisticated data compression techniques and the availability of increased digital signal processing capabilities make it possible to transmit much more high quality audio and video information with digital transmission in the same bandwidth as analog channels. In the television broadcast community, the Advanced Television Systems Committee (ATSC) created both the digital television (DTV) standard and the high-definition television (HDTV) standard to take advantage of these technological advances. These standards are often referred to as ATSC digital TV or simply ATSC.
In terrestrial digital television broadcasting, ATSC receivers must receive and convert radio frequency signals into the digital domain. The ATSC standard uses the amplitude modulated suppressed-carrier vestigial sideband modulation technique called VSB. The 8-VSB modulation is designed for use in terrestrial xe2x80x9coff airxe2x80x9d broadcast systems while the 16-VSB modulation design is for higher-data-rate cable systems.
Errors in these ATSC transmission signals reduce the broadcast signal quality. Transmission conditions contributing to these errors include, for example, multipath signals and interference (co-channel and adjacent-channel) from other analog and digital television signals. In an effort to reduce the effect of these transmission conditions and improve reception, receiver designs include specialized equipment such as high-gain antennas, filters, and digital signal processing techniques.
Adaptive equalizers are used in ATSC receivers to compensate for linear channel distortions from multipath signal components. Multipath channels occur when there are several propagation paths from the transmitter to the receivers. At the receiver, symbols transmitted over these multipath channels may extend over their allotted time interval for transmission and introduce intersymbol interference (ISI). In analog broadcast systems, such as television, the resulting ISI causes voice echoes and video ghosts while in digital systems they typically cause bursts of errors. For a given receiver, an equalizer attempts to cancel the effects of the multipath by applying an inverse filter (a multiplicative inverse in the spectral domain) to the typically non-uniform filter characterizing the multipath in the channel from the transmitter to this receiver.
In addition to radio broadcast systems, early equalizers were also developed for use in telephone networks. These equalizers compensated for the natural filtering of the telephone lines and corresponding distortion of transmitted data pulses and ISI. Many of these early equalization techniques were extended to radio channels where multipath is the primary source of channel distortion. Because the channel conditions are dynamic and not known ahead of time, adaptive equalizers were developed using parameters based on measurements of the received waveform. These types of adaptive equalizers lock on a single multipath signal and incrementally adjust the equalizer parameters each transmitted symbol time interval.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
In one aspect of the invention, a method of performing equalization on a signal received over a broadcast system, includes providing a set of predetermined symbols that correspond to symbols in a data field synchronization segment of a data field having a plurality of segments wherein a data frame having at least two of these data fields is transmitted over the signal in the broadcast system during a block-time interval, correlating complex samples of the signal with the set of predetermined symbols to extract a corresponding impulse response during the block-time interval, selecting a phase identifying the start of the block-time interval from the correlated complex samples, estimating an equalizer filter using the set of predetermined symbols and an initial data field synchronization segment provided at the start of the block-time interval, updating the equalizer filter using subsequent complex samples of the signal to extract a subsequent impulse response and correlate with a reference segment, wherein the reference segment corresponds to either the predetermined symbol sequence or a partial decision feedback segment.
In another aspect of the invention an apparatus for equalizing a signal received over a broadcast system, includes a predetermined symbol section that stores a set of predetermined symbols corresponding to symbols in a data field synchronization segment of a data field having a plurality of segments wherein a data frame having at least two of these data fields is transmitted over the signal in the broadcast system during a block-time interval, a partial decision feedback section that estimates the sign-bits for symbols transmitted in the signal, a complex corrector that correlates complex samples of the signal with the set of predetermined symbols to extract a corresponding impulse response during the block-time interval and determines the phase identifying the start of the block-time interval, and an estimator that estimates the equalizer using the set of predetermined symbols with an initial data field synchronization provided at the start of the block-time interval and subsequently updates the equalizer using subsequent complex samples of the signal to extract a subsequent impulse response and correlate with reference segment, wherein the reference segment corresponds to either the set of predetermined symbols or a partial decision feedback segment.
A block-adaptive equalizer consistent with the present invention includes one or more of the following uses and advantages. Taking advantage of the ATSC broadcast system transmitting known field synchronization segments every 24.2 millisecond, a measurement of the channel impulse response can be obtained using cross correlation techniques. The estimate of the inverse to the measured impulse response is a channel equalizer filter that can be obtained from the periodic transmission of known frame synchronization segments. The block-adaptive equalizer in this invention extends this cross correlation approach to potentially every transmitted segment resulting in a measurement of the channel impulse response and an updated channel equalizer filter each time a data segment is transmitted every 77.3 microseconds.
Using the block-adaptive equalizer, the collection of multipath signals can be acquired and unlike other designs, it does not depend on any one multipath signal that can fade and cause loss of synchronization in the receiver. Like conventional spread-spectrum rake receivers, correlators within the block-adaptive equalizer provide frequent measurements of the channel impulse response whose inverse provide the channel equalizer filters for this design. These channel impulse responses provided by the correlators are used in implementing a block form of blind equalization necessary for tracking the changing multi-path signals that occur between the frame synchronization segments.
The receiver in this invention synchronizes on block-time intervals of 77.3 microseconds each corresponding to the segment time intervals in the ATSC frame structure rather than synchronizing on a particular multipath signal component in the received waveform. Because the block-adaptive equalizer tracks the collection of multi-path signals in the synchronized block intervals, a receiver implementing this type of equalizer will be more stable and can recover quickly after loss of synchronization.